Electric fuse



June 11, 1935.

H. TJB USSMANN ELECTRIC FUSE Filed Aug. 20, 1932 I N V EN TOR. ///v/? TEUSSMHNN Patented June 11, 1935 UNITED STATES PATENT OFFICE 8 Claims.

My invention relates to electric fuses which when forming part of an electric circuit serve to protect said circuit from the effects of excessive current by melting and automatically opening the circuit when the current therein exceeds in magnitude that value for which the circuit is adapted.

One of the shortcomings of fuses as heretofore made is that they blow too quickly on overloads of comparatively brief duration that would not be injurious to the electric wiring or apparatus the fuse is designed to protect.

It is the object of this invention to provide a fuse having greater time lag.

In the drawing Fig. 1 shows a perspective view of a link embodying my invention. Fig. 2 shows a modification thereof. Fig. 3 shows an enclosed fuse embodying my invention.

Fuse links having two fusing portions of equal characteristics are old in the art. In my new improved fuse I use three fusing portions, the outer two of. which are preferably equal in characteristics while the intermediate fusing portion has difierent characteristics.

Links have also been made with more or less than two fusing portions. My invention can be applied with good results to such links as well.

In my application, allowed as Patent #1374252 I described a fuse with considerably increased time lag characteristics and with unimpaired or improved short circuit characteristics.

If an ordinary two notch fuse link is loaded as described in that patent, it is obvious that the maximum current carrying capacity of the link will be increased due to the decreased resistance in the center of the link as well as the greater heat'radiating surface and heat conduction. In spite of this increased current carrying capacity such a fuse will have a greater time lag on overloads of certain percentages based on its own maximum current carrying capacity than would a link without the loading referred to.

By my new invention, I provide such a link with a fusing section approximately in the center. This fusing section is so designed that fusion will take place in this section on the minimum current which will blow the link but on heavy overloads, one or both of the outside fusing portions will blow before the fusing section referred to has reached fusing temperature.

By this means, the maximum current carrying capacity of the link is reduced without appreciably altering the blowing characteristics of the outer fusing portions. On the higher overloads, it would take the same amount of current about the same length of time to blow the outer fusing portions whether the fuse had a center fusing section or not.

This same amount of current would represent a certain percentage of overload in the one case 5 and a higher percentage of overload in the other. The rating of a fuse is based on the amount of current it will carry continuously. Its time lag is judged by the blowing time at certain overloads expressed in percentage of the rating of 10 the fuse. Consequently, I have increased the time lag of the fuse by diminishing its current carrying capacity.

For example, a certain ordinary two notch link was found to have a maximum current carrying capacity of 64 amperes and the blowing time of this link was found to be At 25% overload 31 seconds average At 66% overload 9.4 seconds average At 108 /3 overload 4.2 seconds average Taking the same link and attaching on each side, between the fusing sections as shown in Patent #l,'774,252, additional metal 1.75 times the thickness of the link, and of a width the same as the link, the maximum current carrying capacity was found to be 82 amperes, and the blowing time on this link on certain overloads based on its 82 ampere carrying capacity was found to be At 25% overload 108 seconds average At 66%% overload 20 seconds average At 108%% overload 4.2 seconds average capacity was found to be 45 At 25% overload 83 seconds average At 66%% overload 26 seconds average At l08 /3% overload 10.6 seconds average It will be noted that with this link the blowing time at 25% overload is somewhat decreased but that at the other overloads mentioned the blowing time is increased. The considerable increase in blowing time at 108 A;% overload is particularly desirable and in commercial practice is found much to be preferred to the additional blowing time at 25% overload obtained from the previous mentioned link.

It may appear at first glance that rather arbitrary overloads have been taken in comparing the above mentioned links. Such, however, is not the case. It will be noted that the maximum current carrying capacity has been set forth in each instance. The national electric code standards require that a fuse carry continuously 10% more than its rating. The manufacturer must also provide for slight commercial variations and as a result the rating of a fuse must be less than its maximum current carrying capacity by more than 10%. A rating 16%% less than the maximum current carrying capacity would be entirely reasonable. I

On this basis, a link having a maximum current carrying capacity of 64 amperes would have a rating of 53.3 amperes. 25% overload on 64 amperes is the same current as 50% overload on 53.3 amperes. 66%% overload on 64 amperes is the same as overload on 53.3 amperes. 108%% overload on 64 amperes is the same as overload on 53.3 amperes.

From this itlzan be seen that the figures given in describing these various tests are in reality 50%, 100% and 150% overload on the rating of the fuse, which figures are ordinarily used in commercial description of fuses. Y

I am aware of the fact that fuse links having more than two fusing portions are old in the art but such links do not provide the highly desirable additional time lag which is obtained from a'link of the type embodied in my present invention. For example, a link the same as the first mentioned was taken and a third fusing portion the same size and shape as the other two fusing portions was added at the center. It was found to have a maximum current carrying capacity of 54 amperes and the blowing time on this link on certain overloads based on its 54 ampere carrying capacity was found to be At 25% overload At 66%% overload, At 108%, overload Comparing these results with those obtained on the ordinary two notch link it will be seen that at the first two overloads the blowing time is decreased and at the highest overload is increased practically not at all.

If a current is passed through a homogeneous link of uniform cross-section throughout, the heat generated will be uniform throughout its entire length, but the temperature varies due to conduction of heat by the link and contacts, the center becoming the Hottest for this reason. Even where the center portion is of greater crosssection than other parts of the link, it may reach a higher temperature because of this conduction. This is shown in the case of the two notch link already well known to the art. The heat generated in the weak spots is greater than that generated in an equal length at the center of the link but on a light overload it will be found that the link is blown at the center. I

This is due to the fact that although greater heat is generated at the weak spots, this heat is conducted away by the terminals with sufiicient rapidity to prevent the temperature rising in the 25 seconds average 8.2 seconds average weak spots as rapidly as at the center. Naturally f the heat can play but a small part and as a 4.6 seconds average I result, the fuse blows at one or both of the weak spots and not at the center.

In building my new improved fuse, I bring about the desired result by the proper proportioning of the dimensions of this additional fusing section with reference to the other dimensions of the link and by the disposition of the "loading metal adjacent to this fusing section. These are so proportioned and arranged that when a current is first passed through the link, the conduction of heat from this fusing section is temporarily greater than from the other fusing portions. As the metal adjacent this fusing section heats up, the conduction into this metal becomes less and less. At the same time the conduction from the fusing portions into the contacts, though smaller at first, diminishes but little, because the reat mass of the contacts causes them to heat very slowly. As a result, the fusing sectionfirst attains fusing temperature on currents which heat the link slowly enough for the full effect of conductivity to be brought into play, but on currents of greater magnitude the fusing portions may blow before the fusing section has been heated 'to fusing temperature. The rating of the link is dependent on the blowing of the fusing section referred to. section to act temporarily as if it had a greater rating than it really has, when higher currents are passed through the fuse, the time lag of the fuse is increased.

In Fig. 1, 5 is the basic link having a fusing section 6 near the center and fusing portions I, 1 near the terminal portions 8, 8. Attached to each side of the basic link between the fusing portions 1, I is a strip of metal 9, attaching being accomplished preferably by welds at the 'points l0, l0.

In Fig. 2, II is the basic link having a fusing section l2 near the center of the link and fusing portions l3, l3 near the terminal portions It is obvious that the metal 9, l5 and 23 needbe on only one side of the link if desired.

The link shown in Fig. 1 presents a simple form of my device. Only one piece of metal on each side and only two welds are necessary with this construction. This not alone results in lower manufacturing cost, but makes for greater accuracy as well. The one piece of metal is of reasonably substantial length so that the welded joints are and can be separated a reasonable distance. At 22 is a fusing section. Therefore slight inaccuracies in the distance separation of these welds will have little or no effect on the link. Where a link of the same length is made up with two pieces of metal for loading of the link, and the four welds are necessary, the distance between each two welds is necessarily very short and inaccuracies because of slight variations might result.

It will be noted that the fusing portions near the terminal portions are relatively short and that the member therebetween is relatively long. Thereby the cross-sectional area necessary in the fusing portions to carry the current is held down Therefore, in causing this fusing to a minimum, so that the amount of metal that 35 held down as much as possible.

While I have'shown'several forms of my invention, I do not desire to be restricted to the exact details of construction noted herein for it will be apparentto those skilled in the art that various modifications can be made without departing from the spirit of my invention. While I prefer to use a fusing portion near each end of the link I may omit one of these end fusing portions. The fusing section may have a different cross-section than the end fusing portions or it may have the same cross-section but be of. a different length. It may be of a different material. Its blowing characteristics may be altered by the attachment thereto of a.different material. My invention may be embodied in a link made of a single piece of metal. What I claim as new and desire to secure by Letters Patent is: V

1. A fuse link having terminal portions, a relatively short fusing portion near each of said terminal portions, said fusing portions having substantially the same intrinsic current carrying capacity, a relatively long intermediate member between said fusing portions, a fusing section having an intrinsic current carrying capacity greater than said fusing portions within the length of said member, said fusing section being situated near the center of the link, said member having portions adjacent the fusing section of thickness greater than the terminal portions, to conduct heat from the fusing section whereby the temperature rise in the fusing section is temporarily retarded/on the passage of an electric current through the link and the fusing section is enabled to carry currents heavy enough to fuse one of the fusing portions for a period of time long enough to permit one of said fusing portions to fuse.

2. A fuse link having terminal portions, a relatively short fusing portion near each bf said terminal portions, a relatively long intermediate member between said fusing portions, a fusing section having an intrinsic current carrying capacity greater than that of said fusing portions, said fusing section being within the length of said member and being adapted to blow on the minimum current which will blow the link, said member having portions adjacent to each end of said fusing section of a thickness greater than said terminal portions to temporarily conduct,heat from thefusing section whereby it may bev caused to carry without blowing a current greater than the said minimum current for a time long enough to allow one of the fusing portions to blow.

3. A fuse link having two fusing portions, an intermediate member longer than a fusing portion having for the greater part of its length a thickness of metal substantially greater than the minimum thickness of the link, and means to reduce the maximum current carrying capacity of the link consisting of a fusing section placedinthe intermediate member, said increased thickness of the intermediate member being adapted to conduct heat away from the fusing section whereby the blowing of said section is retarded.

4. A fuse link having terminal portions, a relatively short fusing portion near each of said terminal portions, a relatively long intermediate member between said fusing portions, a single fusing section having an intrinsic current carrying capacity greater than that of said fusing portions, said fusing section being within the length of said member and being adaptedto blow on the minimum current which will blow the link,

said member having portions adjacent to each end of said fusing section of a thickness substantially greater than the minimum thickness of the link to temporarily conduct heat from the fusing section whereby it may be caused to carry without blowing a current greater than the said minimum current for a time long enough to allow one of the fusing portions to blow.

5. In an electric fuse, a casing, terminals attached to said casing and a fuse strip attached to said terminals having a relatively short fusing portion near one of said terminals, a fusing section having an intrinsic current carrying capacity greater than that of said fusing portion, said fusing section being near the center of said strip and being adapted to blow on the minimum current which will blow the strip, a relatively long portion adjacent each end of said fusing section having a thickness greater than the thickness of said strip to temporarily conduct heat from the fusing section whereby it may be caused to carry without blowing a current greater than the said minimum current for a time long enough 'to allow the fusing portion to blow.

6. In an electric fuse, a casing, terminals attached to said casing and a fuse link attached to said terminals having a relatively short fusing portion near each of said terminals, said fusing portions having essentially the same intrinsic current carrying capacity, a relatively long intermediate member between said fusing portions, a single fusing section within the length of said member, having an intrinsic current carrying capacity greater than said fusing portions, said fusing section being situated near the center of the link, said member having portions adjacent the fusing section of thickness substantially greater than the minimum thickness of the link to conduct heat from the fusing section whereby the temperature rise in the fusing section is temporarily retarded on the passage of an electric current through the link and the fusing section is enabled to carry currents heavy enough to fuse one of the fusing portions for a period of time long enough to permit one of said fusing portions to fuse.

'7. A fuse link having two relatively short fusing portions and a relatively long intermediate member therebetween, a reduced fusing section contained within the length of said intermediate member and an additional strip of metal attached to said intermediate member and spanning said reduced fusing section.

8. A fuse link consisting of a basic link increased in thickness for a considerable portion of its length by additional metal which will not fuse before the basic link is blown, said basic link having a fusing portion with some of said additional metal adjacent each end thereof, said additional metal being adapted to produce a retardation of the temperature rise of saidfusing portion, whereby on heavy currents said fusing portion will remain unfused until fusion has occurred in some other portion of the link.

HENRY T. BUSSMANN. 

